PROJECT SUMMARY Cryptosporidia is a diarrheal pathogen with an important impact on global health. Overall, diarrheal disease is estimated to be responsible for 10% of under 5-year old child mortality, and the recent Global Burden of Diseases analysis of diarrheal mortality found that Cryptosporidia was second only to rotavirus as a cause of death in children under five years old (5). Nitazoxanide, currently the only approved drug for Cryptosporidia infection, is only moderately effective for treatment of immunocompetent patients. Furthermore, it is only equivalent to placebo in severely immunocompromised patients. Clinical presentation of Cryptosporidia infection is variable in severity; some patients experience severe diarrheal illness while some infections remain subclinical. Although the cause for the wide spectrum of disease is poorly understood, several studies have demonstrated long-term clinical implications of both asymptomatic and symptomatic infection. We performed a genome-wide association study (GWAS) on infants in Bangladesh within the first year of life. We evaluated the association between 6.5 million single nucleotide polymorphisms (SNPs) across the human genome and symptomatic Cryptosporidia infection in three independent patient cohorts. This analysis revealed a highly significant statistical association of multiple SNPs in the PRKCA gene and susceptibility to Cryptosporidia diarrhea. Each copy of the risk allele increased the risk of contracting cryptosporidiosis by 2.4 times in the first year of life. The most significant SNP in PRKCA is an eQTL in the GTex database, however, a definitive link between PKC? and Cryptosporidia has not yet been established. The PKC? kinase is a primary regulator of actin and during intracellular invasion, Cryptosporidia induces host cell actin remodeling. Further investigation into how the PRKCA gene variation relates to PKC? function and susceptibility to Cryptosporidia infection will provide insight into the pathogenesis of an important human pathogen to cause disease. We hypothesize that host PKC? is required for Cryptosporidia invasion of intestinal epithelium through remodeling of actin cytoskeleton. Based on this preliminary data, we aim to characterize (i) the role of PKC? in Cryptosporidia invasion of the intestinal epithelium in vitro and in vivo (ii) define the impact of the SNPs on PRKCA expression and downstream function, and (iii) identify if PRKCA expression mediates severity of disease in children. To accomplish this, the research proposal includes active investigation of Cryptosporidia both in the field and at the bench. Here, we propose bridging two approaches for scientific discovery by using molecular tools to examine PRKCA and monitoring clinical outcomes in genotyped human children. The overarching goal of these independent yet interacting aims will be the translational development of host PKC? as a potential target to advance critically needed treatments for cryptosporidiosis.